A variety of methods have heretofore been known for producing silicon that is used as a starting material of a semiconductor or a wafer for solar power generation and, among them, some methods have been already put into practical use on industrial scales. For example, one of them is a method called Siemens' method according to which a mixed gas of hydrogen and a trichlorosilane is fed onto a filament that is heated by flowing an electric current, and the polysilicon is obtained by depositing silicon on the filament relying on the chemical vapor phase deposition method. The exhaust gas discharged from the step of producing the polysilicon based on the Siemens's method contains hydrogen, unreacted trichlorosilane and silane compounds such as monosilane, monochlorosilane, dichlorosilane and tetrachlorosilane which are by-products of the reaction, as well as hydrogen chloride. In the invention, hereinafter, the trichlorosilane and the silane compounds are referred to generally as silanes.
In the process for producing the polysilicon based on the Siemens' method, it is a practice to separate the gaseous components contained in the exhaust gas and recirculate them into the production of the polysilicon. For instance, a patent document 1 discloses a method of producing the polysilicon in which the exhaust gas containing hydrogen chloride by-produced in the step of producing the polysilicon is passed through the following steps; i.e., cooled down to −10° C. or lower to remove part of silanes by condensation, the exhaust gas that has passed through the above step is passed through an active carbon layer to remove silanes in the exhaust gas by adsorption, and the exhaust gas that has passed through the step of removal by adsorption is passed through an active carbon layer having a specific average pore radius to remove hydrogen chloride by adsorption thereby to refine hydrogen in the exhaust gas, and the thus obtained hydrogen is circulated into the step of producing the polysilicon.
According to the above proposed method, the hydrogen chloride is desorbed, by using hydrogen as a purge gas, from the active carbon layer that has adsorbed and held the hydrogen chloride in the step of removing the hydrogen chloride by adsorption. Thereafter, the exhaust purge gas containing hydrogen and the desorbed hydrogen chloride is brought into contact with the acid water such as of hydrochloric acid in a washing tower so that the hydrogen chloride in the exhaust purge gas is absorbed by the acid water, and then the hydrogen chloride is recovered from the acid water.